1. Field of the Invention
The present invention relates to the formation of book blocks and particularly to the sequential separation of folded printed sheets from stacks of such sheets in magazines. More specifically, this invention is directed to a sheet separator for withdrawing individual sheets from a stack of sheets and especially to apparatus for withdrawing signatures from magazines pursuant to the manufacturer of books. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention has been designed for, and is particularly well suited for use in, the formation of book blocks in the course of manufacturing books. In the manufacture of a book it is common to assemble the book block on a collecting conveyor by sequentially withdrawing printed sheets from magazines. Such printed sheets may be in the form of "signatures", i.e., the sheets may each have a "spine" fold and define four pages of the text. In the course of book block formation, a plurality of serially arranged separating devices are employed for gathering the printed sheets. Each of the separating devices will typically have a pull-off drum which is located between a magazine and a supporting table which is associated with the collecting conveyor. The pull-off drum supports a gripping mechanism which engages an edge of the lowermost sheet in the magazine subsequent to that edge having been displaced away from the stack. The displacement is typically a downward deflection which is accomplished through the use of a suction device. When the sheets to be withdrawn from the magazine are in the form of the signatures, the downwardly deflected edge will be the spine fold portion of the sheet.
In the prior art, the sheet separating device has included a front limit stop which in part defines the stack-receiving magazine. This front stop will customarily be provided with "separating points" which support the stack of sheets in the magazine at one side of a discharge opening in the bottom of the magazine. The magazine will also include, in addition to the front limit stop, a pair of lateral guides. In operation, a suction device will be caused to move upwardly through the discharge opening to "engage" the lowermost sheet in the stack in the magazine. After the lowermost sheet is engaged, the suction device will move downwardly and thereby deflect the edge region of the lowermost sheet through the discharge opening to a point where it is engaged by the gripper mechanism on the pull-off drum.
The prior art separating methods and apparatus, as briefly described above, have not had the requisite reliability. This lack of reliable operation, to a large degree, results from the fact that the spine folds of the stacked sheets in a magazine bear relatively forcibly against the front limit stop of the magazine. This relatively forcible contact results from the fact that, while the lowermost sheet is being pulled from the bottom of the stack, the frictional forces between the sheets comprising the stack result in their being pulled in the direction of the front limit stop. As a consequence, feed failures are common since the suction device is frequently unable to overcome the frictional force between the leading edge of the lowermost sheet and the magazine front limit stop. The chance of a misfeed, is particularly pronounced when the sheets to be separated from the magazine are comprised of relatively stiff material.